Method of coating structural material



Patented July 25, 1944 METHOD OF COATING STRUCI'URAL MATERIAL Clyde C.Schuetz, Mount Prospect, 11]., assignor to United States Gypsum Company,Chicago, Ill.,

a corporation of Illinois I No Drawing. Application January 11, 1941,

Serial No. 374,081

2 Claims.

The present invention relates to an improvement in the coating ofabsorbent cementitious articles by means of a liquid glazing compositionwhich contains a .volatilizable vehicle, being particularly applicableto the treatment of set cementitious structural materials such as cementblocks, plates, and particularly that species known in the art asasbestos-cement shingles, slabs, and siding.

One of the particular objects of the present invention is to provide amethod of coating such articles with a glazing composition whichcontains an ingredient or ingredients which are capable of reacting withthe constituents of the cementitious material, for inst 11168 thetherein contained calcium compounds or free lime.

It is therefore one of the objects of the invention to apply such aglazing composition under conditions which greatly accelerate theinitial drying or setting of the applied glazing material, especially asecond coat thereof, so that a second coat of another composition may beapplied shortly after the application of the first coat without therebeing any reaction between the second coat and the constituents of thearticle coated.

A further object of the invention is to speed up the production ofglazed cementitious structural material, such as asbestos-cement sheets,

with a silicate composition containing constituentsreactive with thesilicate to form therewith a water-insoluble and weather resistantglazing or coating on said cement sheets, the process being conductibleat such speed that it will lend itself to a continuous operation, as forexample when the sheets are carried on a moving belt and when the glazeis applied by means of rollers, sprays, or similar coating-applyingmeans.

On of the outstanding features of the present invention lies in thepreconditioning of the cementitious articles, which involves the removalfrom the surface thereof of the therein contained sensible moisture,this being accomplished by any suitable drying operation, which may beconducted either at room temperature, as for example by the use of avacuum, or at more elevated temperature, with or without a vacuum. Themain purpose of the removal of this sensible moisture is to render thecementitious articles highly absorbent and receptive to the theretoapplied coating composition, so that the volatile vehicle thereincontained will be. absorbed in part by the dry constituents of thearticlesfor example, by any thirsty or superdried asbestos fiberstherein-while at the same time there may be suflicient heat in thearticles to cause fairly rapid surface evaporation of the vehiclecontained in the coating composition. It will be obvious that in thecase of relatively thick articles, the articles do not need to be drythroughout; but only the surface to be coated needs to be dried to suchan extent that it can absorb the vehicle from a relatively thin coatingwithout becoming saturated. By sensible moisture is meant that moisturewhich can be detected by sight or touch. Care should be taken, however,that the temperature of the cementitious articles, when the coat ofglazing composition is applied thereto, is below the boiling point ofthe vehicle contained in the coating composition, because it is highlydesirable to avoid the formation of actual bubbles of vapor in thecomposition, as such bubbles have a tendency to leave small blisters orpock marks in the glazing coat; and as the second coat is usually fairlythin, it would be insuflicient completely to cover such imperfections inthe priming coat.

A further advantage in the present invention lies in the fact that thefirst coat may be relied upon also to react with the reactableconstituents in the cementitious material, as for example the thereincontained calcium compounds, so as to prevent any further reaction ofsuch constituents with the subsequently applied coating material.

The fundamental parts of the present invention have already beendisclosed in applicants copending application Serial No. 305,124, filedNovember 18, 1939, and the present application is a continuation-in-partthereof. In the aforesaid earlier application the coating of asbestoscement sheets with a silicate coating composition is described,particular emphasis being placed in said application on the nature ofthe composition that is applied to the sheets, which consists of theheat reaction products of a soluble alkali metal silicate and a compoundselected from the group consisting of zinc oxide, calcium carbonate,high burned magnesium oxide and magnesium carbonate in which therelative proportions of MzOZSiOzIMO are respectively 11.7-26.6, 42.1---

62.7, 19.1-41.2 mole per cent (the totalin any case equaling and inwhich M20 represents the alkali constituent and SiO: the silicaconstituent'of the silicate and M0 the silicate compound. In theseformulas, the letter M stands for metal broadly, and of course in theformula M20 such metal will be monovalent, usually being sodium orpotassium, while in the formula MO the metal obviously has the samevalency of oxygen, namely, 2.

The present invention, however, is distinctly not limited to the exacttype of composition, but is broadly applicable to the coating ofabsorbent cementitious articles, irrespective of the actual compositionof the silicate or other glaze thereto applied.

Applying the principles of the present invention to a specific example,but without wishing thereby in any way to limit the broader aspects ofthe invention, the coating of asbestos-cement shingles with a silicatecomposition containing, for instance, zinc oxide, which will form, onproper heating and baking, a weather-resistant coating in accordancewith the disclosure in the parent application, Serial No. 305,124, ofthe present applicant, will now be described. Asbestos shingles asordinarily made contain approximately 85% of Portland cement and ofasbestos fiber. The Portland cement, in hydrating, frees calciumhydroxide. The shingles are stacked in tight piles for curing. As a.consequence, carbon dioxide from the air has access to only the edges ofthe shingles. Thus, under ordinary conditions in a 28-day curing period,a band about one inch wide around the edge of each shingle is quitethoroughly carbonated, but the major portion of the area still containshigh amounts of free calcium hydroxide. It has been found that if ashingle containing this free lime, and having a moist surface, is coatedfor example with a zinc oxide-sodium silicate coating as describedpreviously, it will upon drying develop light-colored streaks within thearea that has not been carbonated by contact wiht the carbon dioxide ofthe atmosphere. No baking treatment will remove these streaks.Furthermore, it has been discovered that if the shingle surface is drywhen the coating is applied, the streaks will not always appear duringdrying or baking but will usually appear after a short period uponweather exposure, regardless of the baking conditions.

However, if one dries the surface of the shingle, appplies one coat,allows it to dry sufliciently so that it loses tackiness and the glossresulting from the presence of water, and then applies a second coat,one can, by properly controlling the baking conditions, obtain ahardened coating which withstands weather exposure withoutchange overlong periods. Apparently the first coat serves to react with, and thusfix, the lime. However, before the second coat is applied, the firstcoat should be suificiently dry so that there is no free interminglingof the materials in the two coats. On the other hand, the first coatmust not be dried by used of such sustained elevated temperatures aswould promote the reaction between the zinc oxide, or other reactant,and the. sodium silicate. In other words, a second coat cannot besuccessfully applied over a baked or substantially reacted first coat.The first coat should not be subjected to a temperature of more than 200F., and this temperature should not be held more than the time requiredto remove the free water from the composition.

In actual practice, the material, such as as- ,bestos Portland cementshingles to be treated, is

passed on a cable conveyor through a heated tunnel in which the shinglesacquire a temperature of from about 100 F. to 150 F., whereby also anymoisture is removed from the surface of the shingles. The warm shinglesare then carried under sprays where the first coat of coating com-.position is applied. The heat contained in the the air expands andbreaks free while the coating will still flow and thus heal anyopenings. The shingles on the conveyor, after the drying interval, passunder a second spray booth in which they receive the second coat ofcomposition. The coated shingles are then ready to be baked for thepurpose of rendering the coating weatherresistant.

The above procedure may be modified by passing the asbestos-cementshingles over a suction plate which will draw heated air around andpossibly some through the shingles for the purpose of removing therefromany therein contained sensible surface moisture. The shingles may, bythis operation, be heated to a temperature of anywhere from 100 to 185F.; or alternatively, and without using the suction means, the shinglesmay be heated by passing them through a suitable oven at a temperaturewithin the limits just mentioned, the main purpose being to remove, atleast from the surface thereof, all therein contained sensible moistureand at the same time to heat them to a temperature which will causerapid evaporation of the water contained in the coating composition. Aconsiderable portion of this water, however, will be almost immediatelyabsorbed by the thirsty asbestos fibers of the asbestos-cement plate andsome of it will also penetrate into the interstitial spaces or pores ofthe composition. The total eifect is a rapid setting up or drying of thepriming coat, this being occasioned in part by such absorption and inpart by the volatilization of the water. By reason of the fact that thetemperature is still below the boiling point of the vehicle in thecoating composition, in this case the water, no bubbles of steam orvapor will form in the composition; and therefore there will be noblisters or pock marks on the first coat. Of course, if a vehicle isused which contains other volatile materials than water-for example,alcohol, benzol, benzine, gasoline, or the like-the temperature to whichthe shingles or other articles are preheated may be modified inaccordance with the boiling point of the particular vehicle. Thus, whenusing alcohol or benzol, the temperature, should be below the boilingpoint of these materials. 0n the other hand, if one were to use avehicle such as rwlene (xylol), the

temperature to which the shingles or other articles are preheated may beabove the boiling point of water but should still be below the boilingpoint of the xylene. In any event, the main object is to produce a rapidremoval of the volatiles from the coating composition, without howevercausing any constituent thereof actually to reach its boiling point.

A very suitable coating omposition for the purpose of the presentinvention may be that which is fully described in the parentapplication, Serial No. 305,124, such as Example I or Example II,contained on pages 3 and 4 of said application as filed.

After the first coat has been dried, which-if the sensible moisture hasbeen properly removed from the shingles and their temperature has beenproperly adjusted-may require as little as one minute, a second coatingof either the same composition or a suitable other composition may thenbe applied, as by spraying, roller coating or the like, whereafter thethus coated shingles may be baked in accordance with the disclosure inthe parent application Serial No. 305,124.

The present. invention is to be distinguished from those operations inwhich a lacquer or enamel is sprayed upon nonabsorbent metallic surfacesor the surfaces of glass and the like, because such surfaces do not haveany absorbent action; and if they were heated, merely a removal of thevolatile materials in the coatings thereto applied would result byevaporation alone. Inasmuch as these coatings have a definite thicknessand the evaporation must take place therefore only from the surface, itwill be obvious that any undue heating of the surfaces to which thecoating composition is applied would lead to the formation of vaporbubbles in the composition. In the present instance, however, by virtueof the absorbent nature of the cementitious material, particularly whenthe sensible moisture has been removed therefrom, the drying or settingof the applied composition takes place from both sides of the film,namely, that which is absorbed into the material coated and that whichevaporates into the surrounding air.

While it has already been proposed to coat various types of asbestoscement plates with coating compositions, this has always beenaccomplished at ordinary room temperatures; and the advantages of thepresent invention have never before been appreciated and are thereforebelieved to be entirely new.

It will, of course, be obvious that the present invention is applicableto the production of building materials of various kinds and may even beapplicable to the coating of absorbent cementitious articles which aremade from such materials as Sorel cement, calcined gypsum, Keenescement, or other setting materials. It will also be obvious that theinvention is applicable to any absorbent structural material whichcontains constituents which are capable of reacting with the theretoapplied coating material, and therefore the present invention is not tobe limited to the examples herein given but is to be construed inaccordance with the hereunto appended claims.

Furthermore, the invention is not to be limited to the method of theapplication of the coating composition, as this may be done by means ofbrushes, air' sprays, mechanical sprays, roller coating, or even bydipping or laying one surface of the material onto the surface of a massof the coating composition, just so long as a coat of the primingcomposition comes into proper contact with the materials being coated.

It is also obvious that the invention maybe practiced when only a singlecoating is applied,

thesensible moisture will apply. As previously mentioned, thepreconditioning of the surface with means for locking up the 'free'lime,such as by treatment with carbon dioxide, may be advisable in suchcases. A singlecoat may thus suffice, being subsequently baked asdescribed or, if the nature of the coating permits, being merely allowedto dry or harden.

I claim:

1. Process of coating asbestos-cement articles I having free calciumhydroxide in the surface because the basic principles of the removal ofthereof with a liquid glazing composition containing a soluble silicateand an insoluble metal compound reactive therewith which comprises thesteps of first removing the sensible moisture contained in the surfaceof said asbestos-cement articles by preheating the same to a temperaturesufficient to remove therefrom the therein contained sensible moisture,thereby rendering the same highly absorbent and bringing them to atemperature which will cause rapid evaporation of water thereto appliedbut without causing sa'd water to boil, then applying a priming coat ofsuch glazing composition to the preheated articlesand causing said coatto dry by the evaporation of the therein contained water as well as bythe absorption of some of such water by the superdried fibrousconstituents of the asbestoscement articles to form a layer comprisingsaid soluble silicate and insoluble metal compound which will not freelyintermingle with subsequently applied coatings, then applying a secondcoat of such composition, completely drying both coats and baking tocomplete the insolubilization reaction between the soluble silicate andsaid insoluble metal compound.

2. Process of coating asbestos-cement shingles having free calciumhydroxide in the surface thereof with a liquid silicate glazingcomposition containing soluble silicate and an insoluble metal compoundreactive therewith which comprises the steps of heating said asbestoscement shingles to a. temperature not exceeding F. so as to remove allsensible moisture from at least the surface thereof and to render themhighly absorbent, then applying thereto a priming coat of said glazingcomposition and effecting the evaporation of the moisture contained insaid composition by absorption of a portion of said water by the fibrousconstituents and interstices of the asbestos-cement shingles as well asby evaporation of the water therefrom to form a layer comprising saidsoluble silicate and insoluble metal compounds which will not freelyintermingle with subsequently applied coatings, then applying a secondcoat of said glazing composition, completely drying both coats, and thensubjecting the thus treated shingles to a baking operation to completethe insolubilization of the compound formed between the soluble silicateand said aforementioned insoluble metal compound.

CLYDE C. SCHUETZ.

